Unless otherwise stated, we assume a flat \(\Lambda \) CDM cosmology with a
Hubble constant \(H_0 =\)  70 km s\(^{-1}\)  Mpc \(^{-1}\) , total matter density
\(\Omega _m =\)  0.27 and dark energy density \(\Omega _\Lambda =\)  0.73. Magnitudes
throughout this paper refer to the AB magnitude system
[1]}.
